1. Field of the Invention
This invention relates to internal speed reduction type starters, and more particularly to planet gear type speed reduction starters comprising planet gear speed reduction means.
2. Background of the Invention
One example of the conventional planet gear type speed reduction starter is as shown in the half section view of FIG. 1. At the end face of a yoke 2 of a DC motor 1, an internal gear 3 of resin forms a planet gear speed reduction means. The internal gear 3 and an intermediate bracket 4 together with a rubber ring 5 are fitted on an inner lower side 7 of a front bracket 6. Further in FIG. 1 is shown an armature 8 of the DC motor 1. An armature rotary shaft 9 has a spur gear 10 at its front end portion which is engaged with a planet gear 11. A bearing 12 is coaxially inserted into the planet gear 11 and is mounted on a supporting pin 13. A flange 14 fixedly secures the supporting pin 13, the flange 14 forming an arm of the planet gear speed reduction means and being integral with an rotary output shaft 15. A sleeve bearing 16 is fitted into the central cylindrical hole of the intermediate bracket 4, the bearing 16 supporting the rotary output shaft 15. A sleeve bearing 17 is fitted into the central cylindrical recess 18 of the rotary output shaft 15, the sleeve bearing 17 supporting the front end portion of the armature rotary shaft 9. Further in FIG. 1, a steel ball bearing 19 is set between the armature rotary shaft 9 and the rotary output shaft 15 and transmits thrust load between these rotary shafts 9 and 15. A thrust washer 20 is placed between the armature rotary shaft 9 and the rotary output shaft 15. Helical splines 21 are formed on the rotary output shaft 15 so that an unillustrated over-running clutch (including a pinion) is slidably engaged with the helical splines 21.
The operation of the conventional planet gear type speed reduction starter thus constructed will be described. The armature 8 produces torque transverse to the drawing when energized, and the torque thus produced is transmitted to the armature rotary shaft 9 and through the spur gear 10, the planet gear 11 and the supporting pin 13 to the flange 14. That is, the speed of rotation of the armature 8, being reduced by this planet gear speed reduction mechanism, is transmitted to the rotary output shaft 15.
In the conventional planet gear type speed reduction starter thus constructed, the internal gear 3 is made of resin (Nylon). Therefore, when a shock occurs at the starting of the engine, the internal gear 3 tends to flex to absorb the shock. However, the internal gear 3 is fitted in the front bracket 6 with a small gap in the radial direction. Therefore, when a shock caused by the ordinary variation in torque of the engine is applied to the internal gear 3, the amount of flexure of the internal gear 3 is so small that the internal gear 3 cannot absorb the shock.
Another example of the conventional planet gear type speed reduction starter in which the gap between the internal gear 3 and the front bracket 6 is excessively large in the radial direction suffers from the following difficulties. That is, when, at the starting of the engine, the engine stops abruptly or the pinion of the starter is not satisfactorily engaged with the engine ring gear, a large shock is given to the internal gear 3. However, in this case, the internal gear 3 is deformed excessively in the radial direction so that the internal gear 3 may be broken.
A similar type device is disclosed in U.K. Pat. No. 2,109,893B which corresponds to U.S. application Ser. No. 435,190 filed on Oct. 19, 1982 and issued as U.S. Pat. No. 4,494,414. In the device, an internal gear is held by a resilient member in the radial direction so that impacting force applied to the internal gear is absorbed by the resilient member. Further, U.K. Pat. No. 2,108,627B discloses an internal gear held by a resilient member in its axial direction. Even if rotational force is applied to the internal gear, its rotation can be eliminated. Even if impacting force exceeding a predetermined level is applied to the gear the force is absorbed by the resilient member. In these two patents, the internal gear is held elastically to absorb impact applied to the gear. However, these patents do not disclose the internal gear having its own resiliency to provide its resilient deformation.